Metal structures immersed in aqueous electrolytic environments are susceptible to corrosion. Corrosion occurs when the current discharges from the metal surface into the electrolyte, thereby creating anodic areas. Cathodic protection is applied to the metal structure with a view to forcing the entire structure surface exposed to the electrolyte to collect protective current. When this environmental condition has been attained, the structure's entire exposed surface becomes cathodic, and corrosion may be sufficiently mitigated, controlled, or prevented. The structure's surface can be made a cathode by means of an externally impressed direct current and/or an attachment to sacrificial anodes. The effectiveness of such cathodic protection can be assessed by measuring the potential of the structure relative to a reference electrode. However, such measurements do not provide an accurate indication of the degree of cathodic protection in a remote or concealed area, such as within a crevice.
The term cathodic protection refers to a process whereby polarized alkaline film at the metal surface is produced by a current density. The quality of this film can be determined by, for instance, measuring it pH value, capacitance, conductance, and polarized potential across the film. However, the protective current density maintaining the polarized surface alkalinity varies significantly with kinetic stress and energy including, temperature, pressure, and motion. The efficacy of the process is determined by the current density requited to maintain the polarized level of protection, and/or the rate of depolarization upon current interruption. When the entire immersed surface becomes equally polarized a potential more negative than −620 mV to the standard hydrogen reference electrode and to the extent that a uniform film (of a thickness in angstroms) with an alkalinity of pH value of about 11.5+/−0.5 is produced, corrosion may then be prevented. Measurements of the structure's polarized surface potential are only conventionally obtained at a considerable distance from the structure. Such remotely measured potentials are, however, only indicative of the average polarized potential over a significant area. Further, such measurements of polarized potential are only a secondary indicator of the alkalinity of the protective film which is the primary means to prevent corrosion and, therefore of the efficacy of cathodic protection. Conventional auto-potential control of impressed current sources alone, therefore, is insufficient unless complemented by pH control compensating for kinetic affects such as temperature and unless the potential pH values are sufficient to stifle microbiologically influenced corrosion (MIC).